An enzymatic sensor for toxic gases is disclosed in U.S. Pat. No. 4,525,704 wherein an enzyme (acetylcholinesterase, AcHe) is covalently bound to a hydrophilic carrier. The enzyme reactant is contained in a pouch and, via the ambient air, is hydrolyzed by the enzyme acting as a catalyst. The reactant is supplied continuously to an enzyme carrier via a filter-membrane combination so that the hydrolysis can take place uninterruptedly as long as no cholinesterase inhibitor is in the ambient to be investigated which would deactivate the enzyme reaction. The hydrolysis is converted to an electrical measuring signal in that the hydrolyzed reactant is oxidized at a measuring electrode.
In this known sensor, acetylthiocholine perchlorate is oxidized to acetic acid and thiocholine perchlorate. The current measured between the measuring electrode and a counter electrode is a measure of the hydrolysis. The enzyme reactant as well as the buffered electrolyte required for the oxidation are contained in a supply reservoir and, via a wick, continuously reach the reactant pouch during operational readiness and measuring time and resupply the reactant pouch with that which has been consumed.
The known sensor tests the ambient air for the presence of such substances which inactivate the enzyme reaction. Such enzyme inhibitors can have different compositions (there are several cholinesterase inhibitors in the cited class of organic phosphorus compounds) so that different inhibitors can be detected only in an unspecific manner by the known sensor. The substance to be detected is itself not the specific enzyme reactant; instead, the enzyme reactant is continuously resupplied by the sensor's own supply means.